Long-Range Order inβ-Brass

The long-range order in $\beta$-brass has been determined over the entire range of temperatures below $T_c$ from x-ray measurements of integrated intensities of superlattice Bragg reflections. Analysis of the data included allowance for the observed appreciable differences between Debye-Waller factors. The normalized long-range-order parameter $\frac{S}{S_0}$ is found to be consistently significantly greater than that predicted by current Ising-model calculations, with the difference increasing monotonically from 1% at $\frac{T}{T_c}=0.\mathrm{}5 \mathrm{to} 13\%$ at $\frac{T}{T_c}\simeq 1$. In the critical region the difference is found to be due to a larger-than-predicted value for the asymptotic coefficient, while the critical exponent is $\beta =0.\mathrm{}300$, in good agreement with the compressible-Ising-model value.